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ADS901E Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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ADS901E Datasheet(HTML) 9 Page - Burr-Brown (TI) |
9 / 11 page 9 ® ADS901 R 1 1k Ω OPA680 V IN R F 402 Ω V CM C 1 0.1µF 0.1µF IN CM +5V R S 50 Ω +3V R G 402 Ω C G 0.1µF 402 Ω 22pF ADS901 FIGURE 4. DC-Coupled Interface Circuit for +3V Single-Supply Operation. dual supplies (OPA650, OPA658). The mid-point reference voltage, VCM, biases the bipolar, ground-referenced input signal. The capacitor C1 and resistor R1 form a high-pass filter with the –3dB frequency set at f–3dB = 1/(2 π R1 C1) (2) The values for C1 and R1 are not critical in most applications and can be set freely. The values shown correspond to a frequency of 1.6kHz. Figure 3 depicts a circuit that can be used in single-supply applications. The mid-reference voltage biases the op amp up to the appropriate common-mode voltage, for example VCM = +1.5V. With the use of capacitor CG the DC gain for the non-inverting op amp input is set to +1V/V. As a result the transfer function is modified to VOUT = VIN {(1 + RF/RG) + VCM} (3) Again, the input coupling capacitor C1 and resistor R1 form a high-pass filter. At the same time the input impedance is defined by R1. Resistor RS isolates the op amp’s output from the capacitive load to avoid gain peaking or even oscillation. It can also be used to establish a defined bandwidth to reduce the wideband noise. The recommended value is usually between 10 Ω and 100Ω. DC-COUPLED INTERFACE CIRCUIT Many systems are now requiring +3V single supply capabil- ity of both the A/D converter and its driver. Figure 4 shows an example for DC-coupled configuration operating solely on a +3V supply voltage. The OPA632 provides excellent performance in this demanding application. Its wide input and output voltage ranges, an low distortion, supports the ADS901 well. The OPA632 is configured for a gain of +2. The 374 Ω and 2.26kΩ resistors at the input level-shift V IN so that VOUT is within the allowed output voltage range when VIN = 0. The input impedance of the driver circuit is set to match to a 50 Ω source impedance. The input level- shifting was designed that VIN can be between 0V and 5V, while delivering an output voltage of 1V to 2V into the ADS901. Both the OPA632 and ADS901 have a power- down function pin with the same polarity for those systems the need to conserve power. EXTERNAL REFERENCE The ADS901 requires external references on pin 22 (REFT) and 24 (REFB). Internally those pins are connected through a resistor ladder, which has a nominal resistance of 4k Ω ( ±15%). In order to establish a correct voltage drop across the ladder the external reference circuit must be able to typically supply 250 µA of current. With this current the full-scale input range of the ADS901 is set between +1V and +2V, or 1Vp-p. In general, the voltage drop across REFT and REFB deter- mines the input full-scale range (FSR) of the ADS901. Equa- tion (4) can be used to calculate the span. FSR = REFT - REFB (4) Depending on the application, several options are possible to supply the external reference voltages to the ADS901 without degrading the typical performance. FIGURE 3. Interface Circuit. Example using the voltage feedback amplifier OPA680. OPA632 V IN 750 Ω 562 Ω 2.26k Ω 374 Ω 22pF +3V 100 Ω Pwrdn DIS Disable +3V ADS901 10-Bit 20Msps 57.6 Ω |
Número de pieza similar - ADS901E |
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Descripción similar - ADS901E |
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